The invention relates to a digitally switchable current source.
In electronic circuits, constant current sources are often required, whose output current must be corrected to a given value after the manufacturing process or whose output current must be varied within given limits.
Inter alia, controllable current sources whose adjusting transistors are driven by an analog control voltage or constant current sources with switchable output stages are known.
A general problem in circuits with switchable constant currents is the generation of current peaks at the switching instants, which peaks lead to damage of the components used or may affect the function of a subsequent circuit arrangement.
It is an object of the invention to provide a digitally switchable current source for arbitrary current values at which no dangerous current peaks are produced at the switching instants.
According to the invention, the object is solved by the characteristic features of claim 1. Further appropriate embodiments are defined in the dependent claims.
In accordance therewith, two digitally controllable switching transistors are arranged in series with at least two parallel-arranged transistors which are controllable by means of an analog voltage, while an element for signal inversion is arranged between the control electrodes (gates) of said switching transistors, one of the two switching transistors being situated in the current circuit whose current is to be controlled by the current source. The parallel, second inversely controlled switching transistor is used for suppressing the current peaks, referred to as spikes, at the switching instant.
The invention has the advantage that an output current of a defined value can be generated. The current source may comprise an arbitrary number of individual constant current sources with the associated switching transistors. The ratio between these current sources may be weighted differently, for example, in a binary form or digitally. The value of the generated current results as a sum from the actively switched paths.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.